The present invention relates generally to current limiting circuits and more particularly to current balancing in circuits of this type.
It is quite common to employ current limiting circuits in high power electrical systems. One such circuit is illustrated in FIG. 1 which has been designated as "prior art". As illustrated in this figure, the circuit, which is generally designated by the reference numeral 10, is connected between an AC power supply (VAC) including source impedance Ls and Rs and a load L and R. This circuit includes a pair of substantially identical parallel connected circuits 12 and 14, each of which includes series connected capacitive and inductive reactance L1, C1 and L2, C2 and resistors R1 and R2, respectively, tuned to resonate at a given frequency. This circuit arrangement also includes a third circuit 16 which is connected across the parallel circuits and which includes a resistance R3 and a switch S.
Under normal operation, the circuit 16 just described should have no significant effect on either the supply or the load. In this situation, the voltage supply VAC supplies power through its source impedence Ls and Rs, through the two circuits 12 and 14 (note that S is open) and finally to the load L1 and R1. If these circuits are indeed equal and if they are tuned to resonate at the supply frequency, there is only a small voltage drop across the limiter 10 caused by the small resistances R1 and R2 which are actually the inherent resistances of the inductors. Moreover, the current through circuit 12 is equal to the current through circuit 14, again assuming that the two circuits are identical. Assume now that a short circuit occurs at the load, simulated for example by closing the switch S1 which is connected across the load. Obviously, in and by itself, this condition would cause a rise in the current through the parallel circuits 12 and 14. To prevent this from happening, the switch S closes in response to the short circuit and the damping resistor R3 is interconnected between the two resonant circuits, thereby taking these circuits out of resonance and causing their impedance to rise to a relatively high value which, in turn, serves to limit the short circuit current.
The particular way in which the circuits 12 and 14 are taken out of resonance, that is, detuned, is of course not limited to the particular circuit 16 illustrated in FIG. 1. This can be accomplished in a number of conventional ways, including some illustrated in U.S. Pat. No. 3,418,532 which discloses a number of current limiters and different types of switching members including, for example, an inductive choke, a surge voltage arrester, spark gaps, or voltage-dependant resistors.
From a practical design standpoint, it is extremely important that the currents through the two paths 12 and 14 in the circuit arrangement described above remain equal or nearly equal under all conditions, and that their arithmetic sum is approximately equal to the line current during normal circuit operation. More specifically, by designing the circuit arrangement to accomplish this, the components of each circuit 12 and 14 can be identical and the necessary "ratings" of these components can be at a minimum. In this regard, it is important to note that a circuit, even though in balance initially, can become unbalanced through for example a malfunctioning component, unless the circuit design anticipates this. If in the design of the arrangement it is anticipated that the currents will not remain substantially equal, due for example to malfunction of individual components, then it is necessary in the initial design to compensate for this, which means providing components in both circuits with substantially higher ratings. This is because it may not be known which of the circuits will carry the higher current. For example, in a typical 300 MVA, 1200 ampere system using a nominal reactance value of 14 ohms and a coil resistance of 0.14 ohms, if one branch has 5% surplus capacitive reactance and the other has 5% surplus inductive reactance, a parallel resonance occurs and substantial voltage is dropped across the current limiter. Hence, the currents through the individual circuits 12 and 14 in this case would be 3060 amperes instead of the nominal 600 amperes. In view of the foregoing, the importance of current balancing in the circuit arrangement of the type described should be apparent. One typical way in which this is done is to employ switching equipment to trim the reactive values to keep the circuits balanced. While this design technique is feasible, because current balance is very sensitive to parameter variations, the switching equipment must be extremely precise. This is difficult to do even where the current balance is made less sensitive to component variations by designing the parallel branches to be slightly off resonance under normal operation.
Another balancing scheme is illustrated in FIG. 2 and utilizes two current balancing transformers M1 and M2 in conjunction with a circuit arrangement otherwise identical to previously described arrangement 10. As illustrated in FIG. 2, this prior art scheme places one winding of transformer M1 and one winding of transformer M2 in series with the components making up circuit 12 and the other windings of these transformers are placed in series with the components making up circuit 14. In this way, the transformers can continuously balance the current through the two circuits. These transformers separately balance the inductor and capacitor currents to accommodate the different phase relationships that exist in the current limiting mode. One transformer, balancing either inductor or capacitor current would also balance total current through the circuits. A disadvantage to this scheme is that the transformers must carry the total rated current and hence must themselves be rated accordingly.
As will be seen hereinafter, the present invention provides reliable current balancing in a current limiting circuit arrangement in an uncomplicated and economical manner. This is accomplished without the disadvantages inherent in the utilization of in-line transformers and yet more accurate than has been accomplished by utilizing external techniques such as the switching equipment described previously.